Deduplicate URLs in record drivers

Details

Description

Currently, the SolrDefault / SolrMarc record drivers make no effort to deduplicate URLs; they simply return whatever they encounter. There might be benefits to making this behavior smarter, but it is also somewhat complex due to the fact that one URL might be accompanied by multiple labels.

A pull request was opened at https://github.com/vufind-org/vufind/pull/434/files to approach this problem, but it needed further work and was eventually closed due to inactivity. That might serve as a starting point if somebody wishes to revisit this issue.